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Read upper and lower deviations and calculate size limits

Convert micrometres to millimetres, calculate size limits and tolerance width from assumed deviations, and check a hole or shaft lookup.

By LinkReady · Updated

Separate sizes from signed deviations

The nominal size is the reference for deviations. Upper and lower deviations are signed differences from it: commonly ES/EI for a hole and es/ei for a shaft. Upper size limit = nominal size + upper deviation; lower size limit = nominal size + lower deviation. Tolerance width is upper deviation minus lower deviation, not the sum of their magnitudes.

An example with two negative deviations

Assume a 25 mm nominal shaft with upper deviation −10 μm and lower deviation −25 μm. These are teaching assumptions, not values assigned to an ISO tolerance class.

−10 μm = −0.010 mm; −25 μm = −0.025 mm

Upper limit = 25 − 0.010 = 24.990 mm

Lower limit = 25 − 0.025 = 24.975 mm

Width = 24.990 − 24.975 = 0.015 mm = 15 μm

Adding the two magnitudes would incorrectly give 35 μm. Mixing μm with mm introduces a factor-of-1000 error.

Preserve the lookup conditions

  1. Identify hole or shaft, nominal size, and the drawing designation.
  2. Check the applicable standard and nominal-size interval, including its boundary conditions.
  3. Convert deviations to consistent units; calculate both limits and their difference.
  4. Compare with the drawing and project requirements, retaining the standard edition and lookup conditions.

Tolerance Master provides hole/shaft tables, tolerance diagrams, and μm/mm displays to help cross-check this process. Read the numbers alongside the diagram.

What the limits do not establish

A shaft meeting its size limits does not guarantee the required clearance with any hole. Calculate both extreme hole–shaft differences; see the clearance, interference, and transition examples. Size limits also do not automatically establish form, orientation, location, or surface-quality conformity.

Terminology reference

The official ISO 286-1 description explains the scope of the system for linear-size tolerances, deviations, and fits. This guide demonstrates arithmetic, not standard-table values. Look up deviations for actual designations in the applicable standard data.